Oblique effects, vertical effects and meridional amblyopia in monkeys

Orientation anisotropies were investigated for monkeys with normal visual acuity and for monkeys with experimentally induced amblyopia. It was found that the majority of control monkeys showed a normal oblique effect if any existing refractive errors were carefully corrected, but a few of the control monkeys had a meridional amblyopia, i.e., an orientation anisotropy in which the grating orientation for the greatest and lowest contrast sensitivities were correlated with the principal meridians of an astigmatic refractive error even when the refractive error was corrected. For monkeys with strabismic amblyopia caused by a surgically induced divergent strabismus, the orientation anisotropies showed a vertical effect in which contrast sensitivity was lower for vertically oriented gratings than for horizontally oriented gratings. However, monkeys with the same degree of amblyopia resulting from experimental procedures that did not involve a misalignment of the visual axes showed orientation anisotropies that corresponded to the usual oblique effect.